HOUSTON, Feb 29 (Reuters) - FPL Group's (FPL.N: Quote, Profile, Research) Florida utility Florida Power & Light said preliminary results of the investigation into Tuesday's four-hour blackout indicated that human error was the primary cause. The blackout affected about 584,000 FPL customers in South Florida, the Juno Beach-based company said in a release on Friday. A total of about 1 million customers of four utilities across the state lost power as emergency safety systems were activated to prevent long-term damage to power plants and transmission equipment. FPL said preliminary findings indicated that a field engineer diagnosing a problem switch at...